Mystery Plant

Well, a mystery to me. It's grown up through the paving slabs on the patio against the wall. I rather like it, but I've no idea what it is. Can anyone enlighten me? Also, is it poisonous?

Note the raindrops on the leaf - yes, we've had rain! It travelled up the A3 with my mother-in-law en route to her lunch date with us. A very nice time was had by all. Now it's back to the books for me.